1. Field of the Invention
The present invention relates to an improved ultrasonic generating and radiating device for use in a sonochemical reactor. More particularly, the invention relates to a device comprising transducers, preferably of the magnetostrictive type, and horns (sonotrodes) that emit ultrasound to the entire volume of a reactor containing liquid, wherein the distribution of ultrasonic energy and acoustic cavitation is homogeneous throughout the reactor volume, leading to an increase in the efficiency of sonochemical processes.
2. Prior Art
Ultrasonic energy has many applications in present-day technology in physical and chemical processes. Some general references are:                1) K. S. Suslick, Sonochemistry, Science 247, pp. 1439–1445 (23 Mar. 1990);        2) W. E. Buhro et al., Material Science Eng., A204, pp. 193–196 (1995);        3) K. S. Suslick et al., J. Am. Chem. Soc., 105, pp. 5781–5785 (1983);        4) Telesonic Co., Products Bulletin.        
This invention relates to a type of reactor in which the reaction occurs in a localized space filled with a material, generally a liquid phase, which may contain solid particles. By the term “reaction” is meant herein whatever phenomenon is caused or facilitated by the ultrasonic radiation, viz. not necessarily a chemical phenomenon, but a physical one or a combination of the two, as well. A reactor of this type is coupled to a transducer, wherein an oscillating, generally alternating, electromagnetic field is generated and an ultrasound emitting means, generally and hereinafter called “horn” or “sonotrode”, receives the ultrasonic vibrations from the transducer and radiates them outwardly into the space occluded by the reactor, hereinafter called “the reaction volume”. The combination of transducer and horn will be called hereinafter, for brevity's sake, “ultrasonic device”. The reactor contains a material to be treated by ultrasound, which will be called hereinafter “reaction material”. The reaction material generally comprises a liquid phase and fills the reaction chamber.
There are several types of ultrasonic reactors. One of them is the loop reactor, described e.g. in D. Martin and A. D. Ward, Reactor Design for Sonochemical Engineering, Trans IChemE, Vol. 17, Part A, May 1992, 29, 3. Inside this reactor, a liquid, which is to be subjected to ultrasonic treatment, is caused to flow in a closed loop formed by a vessel provided with a stirrer and by a conduit in which the ultrasonic generator is housed.
The propagation of ultrasound from a source in an unbounded liquid medium is illustrated in FIG. 2 of the same publication. In this case, the sonochemical active zone is limited to a frusto-conical space diverging from the radiating face of the transducer.
Also, several transducers may be placed around an elongated enclosure, as in Desborough, U.S. Pat. No. 5,658,534 and Caza, U.S. Pat. No. 6,079,508.
The principal drawback of the aforementioned technique is non-homogeneous distribution of ultrasonic energy inside a reaction volume in longitudinal and transversal directions that leads to inefficient sonochemical reactions. The disadvantage is in the limited volume in which acoustic cavitation, hence chemical reaction, takes place.
The application of multiple transducers is used by Dion, U.S. Pat. No. 6,361,747, where multiple transducers are operating at a phase shift from one another, leading to inefficient and non-homogeneous ultrasonic energy coupling that arises from the interference of oscillations with phase deviations.
The purpose of technical solutions described in Dion U.S. Pat. No. 6,361,747 and in Desborough, U.S. Pat. No. 5,658,534 is to create a maximal intensity of ultrasonic oscillations in the center area (that is the area coinciding with the axis of the reactor) leading to a narrow focal zone (cavitation flux) in the center of the volume. The described reactors have a low resonant merit factor because the tube operates in the bending mode of operation and not in the mode of linear oscillations. Such reactors cannot be applied for efficient sonochemical processes, particularly for nano-particle production, which demand an essentially homogeneous distribution of ultrasonic energy throughout the reaction volume.
An additional drawback of the ultrasonic device described by Dion U.S. Pat. No. 6,361,747 is the following: for full energy transmission, it is necessary to provide very tight acoustic contact between ends (edges) of segmental radiators and tube surface, as well as between waveguide and acoustic transducer. The implementation of acoustic contact leads to high-energy losses and to conversion of this energy into high amounts of heat.
The transducers of ultrasound devices can be of various types. Most common transducers are piezo-electric ones. Therein, the generator of the ultrasound typically consists of a piezo-electric element, often of the sandwich type, coupled with a horn having a generally circular emitting face. Piezo-electric transducers, however, have a maximum power not more than 2 kW and a low oscillation amplitude dictated by the fragility of piezo-electric elements, which can be destroyed under prolonged operation. They are also not reliable compared to magnetostrictive transducers, to be described hereinafter, because their amplitude drifts under operation, causing transducer failure and lower energy output, leading to operation parameters that must be manually corrected. Similar properties are also possessed by electrostrictive materials polarized by high electrostatic fields.
Another type of transducer is that based on the use of a magnetostrictive material, viz. a material that changes dimensions when placed in a magnetic field, and conversely, changes the magnetic field within and around it when stressed. When a magnetostrictive material is subjected to an oscillating magnetic field, the material will alter its dimensions at the same frequency with which the magnetic field is alternated.
A magnetostrictive transducer must comprise a magnetostrictive element, e.g. a rod or another elongated element, located in a space in which an oscillating magnetic field is produced. In its simplest form, such a transducer would comprise a nucleus of magnetostrictive element and a coil disposed around said element and connected to a generator of oscillating electric current. However, different forms of transducers can be devised to satisfy particular requirements: for instance, U.S. Pat. No. 4,158,368 discloses a toroid-shaped core of magnetic metal, about which a coil is wound, which toroid defines with its ends an air gap in which a magnetostrictive rod is located.
The ultrasonic transducer transforms the electromagnetic power into ultrasonic power transmitted to an emitting tool—a horn (sonotrode). It will be said hereinafter that the horn emits the ultrasound into a reactor volume, but no limitation is intended by said expression, which is used only for the sake of brevity. Generally, the horns of the prior art have a slim frusto-conical shape or a stepped or exponential shape. In every case, they concentrate the ultrasonic oscillations and emit them from their extremity, which is generally circular and of reduced dimensions. The ultrasonic waves have, therefore, a high intensity only at the extremity of the horn and spread out from it in a conical configuration, so that they reach only certain regions of the reactor volume and at any point of said volume their intensity is reduced, generally in proportion to the square of the distance from the horn extremity. At their area of maximum intensity various phenomena occur, including heating, cavitation, evaporation, and so on, which absorb and waste a large portion of the ultrasonic energy, resulting in a process of low efficiency (ratio of power spent for required process to overall power), which is generally on the order of 20–30%. Additionally, some desired phenomena that are produced by the high energy density at the extremity of the horn may become reversed at a distance from said extremity: for instance, if it is desired to fragment solid particles, contained in a liquid phase, into smaller ones, such smaller particles produced near the extremity of the horn, migrate through the liquid phase and coalesce to some extent at a distance from said extremity, so that the final particles obtained are not as small as desired.
It is a purpose of this invention, therefore, to provide a sonochemical reactor that is free from the drawbacks of prior art ultrasonic devices.
It is another purpose of this invention to provide a sonochemical reactor with substantially homogeneous distribution of ultrasonic energy throughout the volume of the reactor.
It is a further purpose of the invention to provide such an ultrasonic device comprising a transducer that is inexpensive and durable and has a high oscillation amplitude, up to 45 microns.
It is a still further purpose of this invention to provide an ultrasonic device that emits the ultrasonic waves homogeneously in a radial direction, converting longitudinal oscillations into transversal type.
It is a still further purpose of this invention to provide a sonochemical reactor of high power, e.g., up to 5 Kw and more.
It is a still further purpose of this invention to provide a sonochemical reactor, which has at least 60% efficiency, e.g., 60–80%.
It is a still further purpose of this invention to provide a sonochemical reactor, in which there is no occurrence of undesired phenomena at a distance from the horn.
It is a still further purpose of this invention to provide a sonochemical reactor for the effective and high throughput production of nano-scale materials.
It is a still further purpose of this invention to provide a sonochemical reactor for the production of nano-powder materials.
It is a still further purpose of this invention to provide a sonochemical reactor for the production of nano-structured metal powders.
It is a still further purpose of this invention to provide a sonochemical reactor for the production of nano-structured metal oxide powders.
It is a still further purpose of this invention to provide a sonochemical reactor for the production of nano-structured metal hydroxide powders.
It is a still further purpose of this invention to provide a sonochemical reactor for treating agglomerated materials and effecting de-agglomeration.
It is a still further purpose of this invention to provide a means for the acceleration of chemical reactions.